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Caloris Planitia

The most prominent and distinctive feature on the surface of Mercury is a vast circular basin ringed by mountains. The Caloris Basin, or Caloris Planitia, is the result of the impact of an immense object - estimated to have been some one hundred kilometres in diameter - that left a vast circular scar filled with smooth solidified lava. Around the basin a circular range of mountains has been raised up to two kilometres in height, known as the Caloris Montes.

The Caloris Basin is more than 1,500 km in diameter, comparable to the Mare Imbrium on Earth'sMoon, though somewhat larger. Both these features are thought to have had similar origins, being formed early in the history of the Solar System through significant impact events. Though both contain craters of their own, this is much lighter than on the surrounding terrain, implying that these features were formed near the end of the most intense period of bombardment.